Aluminium borates: synthesis via a precipitation process and study of their formation by DSC analysis

Aluminium borates have been synthesized via a precipitation process. Their formation has been studied by DSC analysis and X-ray diffraction. Aqueous solutions of aluminium nitrate and boric acid in various proportions were precipitated into solutions of ammonium carbonate, acting as a base like ammonia. The resulting mixtures were evaporated to dryness in order to recover the soluble boron species (ammonium borate). The by-product ammonium nitrate was decomposed at 300 °C. The DSC analysis, at 5 K min−1, showed the crystallization temperature of aluminium borates to decrease regularly with the increase of the boron content in the sample, from 900 °C for the A9B2 compound (abbreviation of 9Al2O3⋅2B2O3) to 670 °C for a composition “AB2”. A9B2 was stable, at least up to 1200 °C. For boron-richer compositions, like “A6B2”, a boron-excess A9B2 solid solution was crystallized at the detriment of A4B2. This metastable solid solution decomposed at 1000 °C into A9B2 and boron oxide. Even for the composition “A4B2”, A9B2 crystallized along with A4B2. The latter compound was found to be stable up to 1000 °C, and to decompose into A9B2 and boron oxide at higher temperature, without any sign of melting. This decomposition was confirmed by thermogravimetric analysis. The problem of volatilization of boric acid or boric oxide, often encountered in the synthesis of borates, is avoided.